When a dentist extracts a tooth, the procedure leaves behind a small hole where the tooth once was. This socket can be very sensitive at first, which is why your dentist may recommend socket preservation to go along with your extraction.
It's not uncommon for the bone that previously supported the tooth to melt away once the tooth is removed – it doesn't serve a purpose anymore. When this bone fades away, however, gaps form between the teeth, or the teeth can move out of alignment. This is why a socket preservation is typically performed at the same time as the tooth extraction itself.